1. Field of Invention
The invention relates to an exhaust gas emission control system of an internal combustion engine and a purification capacity recovery method of the exhaust gas emission control system.
2. Description of Related Art
Exhaust gas discharged from an internal combustion engine contains harmful substance, that is, NOx. Generally an NOx catalyst is provided in an exhaust system of the engine. In the case where an occlusion/reduction type NOx catalyst is employed, the more the occluded amount of the NOx increases, the less the exhaust gas purifying capacity of the NOx catalyst becomes. For this, a fuel functioning as a reducing agent is supplied to the occlusion/reduction type NOx catalyst so as to release the NOx that has been occluded in the NOx catalyst through reduction. The aforementioned method will be referred to as “NOx reducing method” hereinafter. It is well known that as the NOx catalyst occludes SOx contained in the exhaust gas, it may suffer an S-poisoning, resulting in the deteriorated exhaust gas purification capacity. The fuel functioning as the reducing agent is supplied to the NOx catalyst so as to eliminate the S-poisoning. The aforementioned method will be referred to as “S-poisoning eliminating method” hereinafter.
If the flow rate of the exhaust gas introduced to the NOx catalyst is not appropriate upon supply of the fuel as the reducing agent thereto, there may be the case that a part of the supplied fuel cannot be sufficiently used for oxidation on the NOx catalyst. This may fail to perform the NOx reducing method and the S-poisoning eliminating method (hereinafter collectively referred to as NOx catalyst recovery method), or deteriorate the fuel efficiency.
A publication of JP-A-2003-74328 discloses the art for efficiently using the fuel supplied as the reducing agent for the NOx catalyst recovery method, in which two NOx catalysts are provided in an exhaust emission control system (including exhaust catalytic system such as a NOx catalyst, and the control system therefor), and the flow rate of the exhaust gas that flows through one of those two NOx catalysts is controlled to a predetermined value by a valve capable of changing the cross section area of the exhaust passage so as to supply the fuel as the reducing agent to the NOx catalyst in the passage at which the flow rate of the exhaust gas is controlled. The thus supplied fuel may be efficiently used under the NOx catalyst recovery method.
In the aforementioned art, however, all the amount of the fuel functioning as the reducing agent cannot be fed to the NOx catalyst including its downstream portion. This makes it difficult to perform the NOx catalyst recovery method efficiently, especially when the NOx catalyst is substantially long in the flow direction of the exhaust gas, or a plurality of NOx catalysts are provided in series.